Identifying Lithium Percolation Network Connectivity Induced by Fluorination in Mn-Based Disordered Rocksalt Cathodes

Abstract Cation-disordered rocksalt (DRX) cathodes can achieve high reversible capacities using earth-abundant transition metals, where Li transport is governed by a percolation network. Fluorination modifies the local short-range order, influencing the 0 TM percolation. In this study, we controlled the synthesis parameters, including F content, sintering temperature, and sintering duration, for Mn-based DRX with a formula of Li1.2Mn0.5Ti0.3O2–xFx (LMTOF, x = 0, 0.05, 0.1, and 0.2) to improve the percolation network. Raman analysis revealed that fluorination enhanced the structural coherence of Mn–O bonding, as evidenced by the narrowing of the A1g and Eg bands and the reduced ratios of Eg/A1g and A1g-Hi/A1g. Also, a gradual decrease in the Mn–O/F peak from extended X-ray absorption fine structure (EXAFS) analysis indicates the successful substitution of O by F. Low F levels (x < 0.1) can hinder Li percolation via the Li–F gathering effect, whereas high F levels improve Li percolation, reducing overpotential and improving Li diffusion at the optimal fluorination level. Furthermore, ex situ X-ray diffraction (XRD), Raman spectroscopy, and scanning transmission electron microscopy (STEM) demonstrated that optimal fluorination facilitated continuous and reversible structural transformations with enhanced kinetics. Overall, this study establishes the relationship between structural evolution and electrochemical performance to optimize the fluorinated DRX cathodes.